In the related art, optical films including polarizing films are used in image display apparatuses such as liquid crystal display apparatuses.
As a method of producing such a polarizing film, a process is used in which a polyvinyl alcohol resin film (raw film) is fed from a raw film roll produced by winding a band-like polyvinyl alcohol resin (PVA) film as a raw film into a roll; and the polyvinyl alcohol resin film is stretched by passing the film through an apparatus including a plurality of rollers for restricting the traveling path of the raw film and guiding the raw film and a variety of chemical solution baths. For example, a process is used in which the raw film is traveled in the longitudinal direction, and dipped into a swelling bath and a dyeing bath successively; then, the raw film is nipped by the rollers at two places of a leading portion and a tail portion of the raw film in the traveling direction, and a tension is applied therebetween to stretch the raw film.
In the method of producing such a polarizing film, every time when the raw film roll is replaced, a new raw film is wound around the rollers or the like, and set in the apparatus. This is very complicated and time-consuming. For this reason, the leading end of the raw film fed from the next raw film roll is bonded to the tail end of the previous raw film, and the two joined raw films are sequentially and continuously processed into a polarizing film.
As such a bonding method, in the related art, methods such as adhesive bonding methods using an adhesive tape, an adhesive or the like, sewing bonding methods using a rivet, a thread or the like, or heat melting bonding methods using a heat sealer or the like, are used.
Such methods, however, have problems below.
Problems in Adhesive Bonding Using an Adhesive Tape, an Adhesive or the Like
In the step of dipping the raw film in a swelling bath, a dyeing bath or the like, the component of the adhesive and the like are eluted into the chemical solution to contaminate the chemical solution. The contamination may cause adhesion of foreign substances to products. In addition, the adhesive dissolves in the chemical solution or swells due to a component in the chemical solution to reduce bonding strength. This may break the joint portion in the stretching step before the stretch ratio reaches a desired stretch ratio.
Problems in Sewing Bonding Using a Rivet, a Thread or the Like
In this method, holes for a rivet or a thread are formed in the raw film. For this reason, when a tension is applied to the joint portion, the joint portion may be broken from the holes.
When the number of holes is reduced and the holes are formed at a larger interval in order to prevent such breakage, application of a tension may easily cause wrinkles, leading to uneven stretching.
Problems in Heat Melting Bonding Using a Heat Sealer or the Like
As a bonding method that can solve the problems in the adhesive bonding and the sewing bonding, a bonding method using a heat sealer is known as shown in Patent Documents 1 and 2 below.
In this method, the chemical solution is less contaminated than in the case of the adhesive bonding, and no holes need to be formed unlike the case of the sewing bonding.
It is difficult, however, to heat only a narrow area with a heat sealer, and a relatively large area of a welded region is likely to be formed. The welded region and portions around the welded region tend to be modified by the heat applied during welding and become harder than other normal portion.
For this reason, when a tension is applied with the welded region being nipped during stretching, distortion is likely to be produced intensively at a boundary portion between the hardened portion and the other normal portion, and the region may be extremely stretched before the whole nipped portion of the raw film reaches a desired stretch ratio.
Accordingly, when the raw film is stretched at a high stretch ratio, the joint portion of the raw film may be broken.